Elevated operating speed of central processor units of electronic computers has demanded corresponding speed-up of high speed printing machines, which are an output unit, and various improvements have been made to meet such demands.
A typical example of such improvements is a system in which ink is supplied to the nozzle under a very low pressure so that ink bulges out half-spherically from the nozzle end, then ink is drawn out in the form of droplets by giving an electric field between said nozzle and an accelerating electrode placed several millimeters ahead of the nozzle, and then the ink droplets are directed toward the surface of the printing paper by giving a strong electric field between the nozzle and a back bar and further electrostatically defected in both primary and secondary directions (right and left directions) to thereby print letters, signs, etc., on the surface of the printing paper.
There is also known a system in which the ions produced between the high voltage impressed electrodes are passed through an ink mist so that they carry therewith the ink particles, and then these ink particle-carrying ions are selectively adsorbed on the surface of the printing paper to thereby effect printing.
The present invention pertains to the last-mentioned system, and so this system is first described for facilitating an understanding of the present invention.
Referring to FIG. 1 which diagrammatically depicts the said system, a high voltage is applied between the anode 101 and cathode 102 to produce cations from the anode 101. These cations are attracted toward the cathode 102 and attempt to pass through an aperture board 103. This aperture board 103, as shown in FIG. 2, consists of controlling conductive layers 105, a common conductive layer 106 and an insulating layer 107 interposed therebetween, said layers being formed surrounding each of the openings 104 in the board, and the cations pass the openings 104. A predetermined amount of voltage is applied to both said controlling conductive layers 105 and common conductive layer 106 to form an electric field in each of said openings 104, so that said cations are controlled to pass or so as not to pass the openings by changing the direction of said electric field by properly selecting the potential of the controlling conductive layers 105 with relation to the common conductive layer 106. Said openings 104 are provided a plurality of lines, and the cations which have passed these openings impinge against the ink particles while passing the ink mist to electrically charge said ink particles. The ink mist is generated by vibrating the ink solution 109 in an ink mist generator 108 by supersonic vibrators 110 and discharged out from an outlet 111. The electrically charged ink particles are attracted to the cathode 102 of the back bar 112 and deposited on a printing paper 113 to print dots. In this way, letters, etc., in the form of a dot matrix are printed by controlling the feed of the printing papers 113 as auxiliary scanning. In such electrostatic high speed printers, the aperture board plays an important role. Therefore, various means are provided for protecting the aperture board against failure or damage, and may of such means are designed for preventing blocking of openings through which the ions pass. One of the most effective methods for preventing blocking of the openings is to feed air into the aperture board and then let it eject out from the openings. This method is effective for protecting the openings against dust, ink particles, etc., but it still involves a problem that if moist air is fed, the exposed metallic parts of the controlling conductive layers and common conductive layer could gather rust.